embriyon

An embryo is the early developmental stage of an animal or plant that begins after fertilization and continues through the initial phases of development. In humans and many animals, the term typically refers to the stage from fertilization until the end of the eighth week of gestation; after this point, the developing organism is usually called a fetus. In plants, embryos form after fertilization within seeds and give rise to the seedling.

In the initial stages, a zygote undergoes cleavage divisions to form a multicellular structure called a morula, which develops into a blastocyst. The blastocyst has an outer cell layer (trophoblast) and an inner cell mass that will become the embryo proper. Implantation into the uterine lining marks the beginning of pregnancy. During gastrulation, the inner cell mass differentiates into three germ layers—ectoderm, mesoderm, and endoderm—setting the stage for organ formation (organogenesis). The neural tube, arising from ectoderm, is one of the early signaling events in vertebrate embryos.

Embryos differ among species in timing and specific developmental events, but the general sequence—fertilization, cleavage, blastocyst formation, implantation, germ-layer formation, and organogenesis—is widespread in animals. In plants, the embryo within the seed undergoes similar differentiation to produce the seedling.

Clinical and ethical contexts surround human embryology. Embryos are central to assisted reproductive technologies, such as in vitro fertilization, and to embryonic research, which is governed by regulatory frameworks and ethical guidelines that address conservation, manipulation, and allowable experimentation, including considerations around the duration and use of embryos in research.